The retinoblastoma gene, Rb, is a prototype growth suppressor gene, the inactivation of which is associated with tumor development. The finding that the Rb protein can bind to the transformation proteins of several DNA tumor viruses suggest the existence of cellular proteins that are the structural and/or functional homologs of the viral proteins. In attempt to isolate such proteins, the investigators made the surprising discovery that the Rb protein shares an antigenically homologous domain, P16, with the SV40 large T antigen. They surmise that this structural motif, conserved between Rb and large T, may also be present in the putative cellular homologs of the large T antigen (CHLA) and to other proteins that are structurally related to the RB protein (CHRBs). Therefore, the anti- RB antibody 16-2, which can also recognize large T, was used to screen several lambda gt11 libraries. A number of candidate clones were isolated. Initial sequence analysis of several of the clones revealed yet another domain, P9, that is also conserved between RB, LT and some of the isolated genes. It appears that there are at least three types of genes in the collection of clones they isolated. They are (1) CHLAs; (2) CHRBs; and (3) RB related transcription factors (RBTFs) - a class of genes that is structurally related to RB and to some known transcription factors. Based on these observations, they propose a model in which the Rb protein (and related proteins) controls differentiation and cell growth by regulating the activities of tissue specific transcription factors. It does so by binding to the transcription factors or the modulators in the regulatory pathway through either the P16 and/or the P9 domain. By virtue of the P9/P16 domain, the SV40 large T antigen and its cellular homologues may compete with the Rb protein and its homologues for the transcription factors or their modulators. Therefore, the clones they isolated may represent three different components of the Rb regulated pathways, all interacting via their P9/P16 domain. In the present application, experiments are proposed to test the various aspects of the hypothetical model. Specifically, they will be examining the roles of the clone RBTF-C, the gene product of which shares homology with those of C/EBP and RB-1, in Rb mediated transcription regulation and growth suppression.